Split sector recovery method

ABSTRACT

Reproduction of encoded data which includes a split-mark. FIR data corresponding to split-mark and FIR data affected by the split-mark due to inter-symbol-interference are identified. FIR data corresponding to the split-mark is removed from the received FIR data. Recovered data is created by removing incorrect inter-symbol-interference from the FIR data due to the split-mark, and adding correct inter-symbol-interference from codeword bits. The recovered data is stitched together with data unaffected by split-mark data.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/263,235, filed Oct. 31, 2008, which claims the benefit of U.S.Provisional Patent Application No. 60/986,366, filed Nov. 8, 2007, thecontents of which are hereby incorporated by reference as if fullystated herein.

TECHNICAL FIELD

The present disclosure generally relates to reading data from a harddisk which includes a split sector.

BACKGROUND

Storing and retrieving data on a hard disk or other storage media areessential to modern computing. Ordinarily, data is stored in a hard diskin concentric circles called tracks. The disk is generally furtherformatted with the SERVO wedges. In today's drives there are generallyabout 200 SERVO wedges. Portions of the tracks corresponding to servowedges contain some system information and user data is generallywritten elsewhere on the track.

User data is generally stored on the media in logical sector format,e.g. 512 bytes of user data plus some overhead for error correctioncode. Logical sectors are protected by Error Correction Code (ECC), e.g.Reed Solomon (RS) or Low-Density Parity Check Code (LDPC) to ensure highdata reliability. ECC may protect an entire logical sector or logicalsectors may be sub-divided into multiple ECC code words.

Sometimes it is not possible to fit an integral number of logicalsectors between two consecutive SERVO wedges. In this case, a logicalsector is broken into two physical sectors. A first physical sector iswritten before the SERVO wedge and a second physical sector is writtenafter the SERVO wedge. This is commonly referred to as a split sector.In the absence of a split sector, a logical sector coincides with aphysical sector. Each physical sector generally has the following formaton the media: (preamble (a sequence of 00110011 . . . ,), sync mark 1,user data+ECC, postamble (11001100 . . . )). Alternatively, it is alsopossible to have two sync marks per physical sector: (preamble, sync1,data1, sync2, data2, postamble). Here the second syncmark splits theuser payload into two parts: data1 and data2.

Split sectors, and second syncmark causes fragmentation of user data onmedia in that user data that belongs to the same logical sector does notcorrespond to a continuous segment on the media. Instead some other bits(e.g. corresponding to the 2^(nd) syncmark) are placed in-between userbits. During the readback process, the decoder has to remove any systeminformation that was inserted in-between the data (e.g. sync2) andformat the data into logical sectors before starting an ECC decodingprocess and/or returning data back to a Host.

SUMMARY

The present disclosure addresses the foregoing issues by providing aformatter block that can stitch FIR samples together into a singlecontinuous stream, thereby removing the necessity for an iterativedecoder to have any knowledge of data format on the media. Multipleimplementations of a formatting operation are discussed herein. Inparticular a formatting operation that largely removes any discontinuityin an FIR data stream that might result in stitching FIR samplescorresponding to not-contiguous bits streams on the media is disclosed.

This brief summary has been provided so that the nature of thedisclosure may be understood quickly. A more complete understanding canbe obtained by reference to the following detailed description inconnection with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a representation of a hard disk including a split sectorcaused by a servo wedge.

FIG. 1B is a representational view of a hard disk drive according to anexample embodiment.

FIG. 2 is a block diagram illustrating an encoding process in which asyncmark is introduced into a codeword.

FIG. 3A illustrates an analog waveform without a split sector.

FIG. 3B illustrates an analog waveform with a split sector added.

FIG. 3C illustrates a comparison between the analog waveform without asplit sector, and an analog waveform after simple stitching isconducted.

FIGS. 4A and 4B are block diagrams of a system for split-sectorrecovery.

FIG. 5 is a flowchart illustrating a process of split-sector recovery.

FIG. 6 is a histogram comparing error locations at a soft output Viterbialgorithm (SOVA) detector between data without a split sector, data withsimple stitching, and data reproduced by the system for split-sectorrecovery.

FIG. 7 is a histogram comparing error locations at an outer decoderbetween data without a split sector, data with simple stitching, anddata reproduced by the system for split-sector recovery.

FIG. 8 is a chart of bit error rate (BER) against signal-to-noise ratio(SNR) at a SOVA detector between data without a split sector, data withsimple stitching, and data reproduced by the system for split-sectorrecovery.

FIG. 9 is a chart of bit error rate (BER) against signal-to-noise ratio(SNR) at an outer decoder between data without a split sector, data withsimple stitching, and data reproduced by the system for split-sectorrecovery.

DETAILED DESCRIPTION

A process of recording data onto a disk and common causes of split-markdata will be described with respect to FIGS. 1 to 3.

Generally, to store data on a hard disk, the data is partitioned intological sectors, encoded by Error Correction Code (ECC), and thenwritten onto the magnetic media.

In accordance with various embodiments of the present invention, in theabsence of a split sector and a second syncmark, the entire logicalsector is written to a portion of the track without any system databeing inserted in between. For such sectors, ideal FIR data is justconvolution of codeword bits with the target response h(k) as shown inthe following equation, Eq. (1):

$\begin{matrix}{y_{i}^{\prime} = {\sum\limits_{k = 0}^{M}{h_{k} \cdot c_{i - k}}}} & (1)\end{matrix}$where y′_(i) is ideal FIR data, c_(k) are codeword bits, h_(k) is targetchannel response of form h=[h₀, h₁, . . . h_(M)], and M is the memorylength of channel response.

On the other hand if the amount of media between sector start and nextSERVO wedge is not enough to hold the entire logical sector, then thelogical sector may be split into two (or more) physical sectors.Sometimes it is not possible to fit an integral number of logicalsectors between two consecutive SERVO wedges. In this case, a logicalsector is broken into two physical sectors. A first physical sector iswritten before the SERVO wedge and a second physical sector is writtenafter the SERVO wedge (For example, see FIG. 1A). This is commonlyreferred to as a split sector. In the absence of a split sector, alogical sector coincides with a physical sector. Each physical sectorgenerally has the following format on the media: (preamble (a sequenceof 00110011 . . . ,), sync mark 1, user data+ECC, postamble (11001100 .. . )). Alternatively, it is also possible to have two sync marks perphysical sector: (preamble, sync1, data1, sync2, data2, postamble). Herethe second syncmark splits the user payload into two parts: data1 anddata2.

As can be seen from FIG. 1A, a split sector 103 has been introduced intothe data on hard disk 101. This split sector is caused by a servo wedge104 intersecting encoded data on that sector. A servo wedge bisects atrack 102 causing the split sector 103. In contrast, non-split sector105 consists of continuous data. A similar case occurs if a dual syncmark is used. The second sync mark fragments the ECC codeword into twoparts.

As indicated above, a servo wedge is added to the recorded datapurposely by the system. For example, the servo wedge can contain aunique magnetic pattern that provides a reference to the center of thetrack, so that the hard drive can quickly and accurately locate data onthe disk.

FIG. 1B is a representational view of a hard disk drive according to anexample embodiment. Mechanically, hard disk drive 150 includes housing151 which houses hard disk 101. In one implementation, hard disk 101 isa platter coated with a magnetic material and rotatable about spindle152 by a drive motor (not shown). A read/write head 153 is mounted toarm 154 for movement of the read/write head across the surface of harddisk 101 under control of actuator 155.

Hard disk drive 150 further includes disk controller 156 which generallyincludes a processor such as a digital signal processor, amicroprocessor, a microcontroller or the like, for execution ofinstructions stored in memory for electrical and mechanical control ofthe hard disk drive components, and for electrical and mechanicalcontrol of hard disk drive circuitry. Disk controller 156 includes aninterface via bus 157 to a host 158. Host 158 might be a personalcomputer such as a laptop or desktop, or host 158 can be an embeddeddevice such as a hand-held PDA or music player. Other examples of host158 are given in connection with FIGS. 10A through 10H, below.

Bus 157 can be an EIDE interface, an ATA or serial ATA (SATA) interface,a fiber channel (FC), or a serial attached SCSI (SAS) interface,although it will be understood that other buses and other interfacesmight be used.

FIG. 2 is a block diagram illustrating an encoding process that includesintroduction of a second syncmark 204 into a codeword 205, causing asplit within the codeword 205.

As mentioned above, when user data is stored on a hard disk 206, it isfirst encoded into binary data, a process represented by encoder 201 Acodeword of length N may be represented as:

-   -   c₁c₂c₃c₄ . . . c_(N-3)c_(N-2)C_(N-1)C_(N)        where (c₁ . . . c_(N)) represent the encoded and interleaved        data bits.

However, a second syncmark 204 may be added to the data. For example,the syncmark may be added to aid data synchronization. Thus, thecodeword 205 on the hard disk 206 now contains a ‘split’ due to theaddition of the syncmark data. The written data stream may berepresented as:

-   -   c₁c₂c₃ . . . c_(L)a₁a₂a₃ . . . a_(s-2)a_(s-1)a_(s)c_(L+1) . . .        c_(N-1)c_(N)        where (a₁ . . . a_(s)) represent the split-mark data bits.

As a result of the servo wedge in FIG. 1B or the second syncmarkinsertion in FIG. 2, FIR samples corresponding to a physical sector donot form a continuous stream, and instead contain some samplescorresponding to system bits (e.g. second sync, preamble or postamble).Thus, when the data is read from the disk, the system needs a way todiscard the split-mark data and the accompanying inter-signalinterference (ISI) affecting the data following the split.

One method for removing this discontinuity is referred to as “simplestitching”, in which the FIR data before and after the split are simplyconnected together in the reproduction process. However, simplestitching does not adequately account for the incorrect ISI in thesignal due to the split.

For example, FIG. 3A depicts an analog waveform of data without a splitas it appears when read from a hard disk, and FIG. 3B depicts an analogwaveform when read from a hard disk with an added split where [A] showsthe beginning of the split data and [B] shows the end of the split data.

FIG. 3C shows an FIR waveform without a split and an FIR waveform thathas had a split removed using simple stitching. These waveforms areshown as they appear after reading from the hard disk. As seen in FIG.3C, the simply stitched waveform deviates significantly from the idealwaveform without a split.

Accordingly, a system and method for split sector recovery will now bedescribed with respect to FIGS. 4A, 4B and 5.

FIGS. 4A and 4B schematically depict data reproduction systems in whichsplit sector recovery may be practiced. This system may be embodied inany number of devices for reproducing data, including a hard diskcontroller, a hard disk drive, a CD-ROM drive, and a DVD-RW drive, amongmany others.

In general, a read-back waveform passes from the disk 400 to the analogfrom end (AFE) 402, sampled by the analog-to-digital converter (ADC)404, and passed through a finite impulse response (FIR) filter 406.Output of the FIR filter 406 is referred to as FIR data henceforth. FIRdata of every recorded bit generally contains interference from adjacentrecorded bits, commonly known as inter-symbol-interference (ISI). FIRsamples are fed to a Viterbi detector 408 that produces hard decisions.Due to channel noise, the bit stream at the output of the Viterbidetector 408 might contain errors. The Viterbi output is then given toan ECC decoder 412, after passing through formatter 410, to correct anyresidual errors before outputting the data to the Host.

If the decoder 412 utilizes hard decision decoding (e.g. RS ECCdecoder), then the Read Formatter 410 simply has to remove the bitscorresponding to the second syncmark prior to using the RS ECC decoder412. Similarly for the case of split sector, the formatter 410 has toconcatenate several data streams that come at different times in orderto form a received ECC codeword. A corresponding configuration for LDPCcode is shown in FIG. 4B. If the decoder 412 is an iterative decoder,i.e. the decoder consists of a channel detector 416 (e.g. BCJR/SOVA) anda LDPC code decoder 418 (e.g. Sum-Product (SP), or Min-Sum (MS)decoder), then such a decoder takes FIR samples as the input. It thenproceeds to iterate between the channel and the code decoder until somepredetermined number of iterations have been performed, or no parityviolations have been detected. FIG. 4B illustrates an exemplary blockdiagram of a data path with an iterative decoder 414. Unlike a hard RSECC decoder that takes Viterbi hard-decisions as input, an iterativedecoder takes FIR samples as the input from the FIR Sample Buffers 420.Consequently, in the presence of an iterative decoder, the formatter 410has to group together FIR samples corresponding to an entire iterativecodeword (i.e. remove samples corresponding to second sync, and or anydata added to accommodate split sector.

Note that forming a continuous stream of FIR samples corresponding to aniterative codeword is not necessary. Alternatively, it is possible torun the channel detector on FIR samples that include the second syncmark, obtain Log-Likelihood Ratios (LLR's) and then have a formatterinside the iterative decoder to remove LLR's corresponding to, forexample, the second sync mark prior to sending them to the code decoder.However, such an architecture would be very difficult to implement andwould require extra storage for FIR samples corresponding to split-markdata. Furthermore, having to process extra samples (ones correspondingto second sync or split-mark data) would unnecessarily eat up availabletime, and therefore result in a reduction of the number of iterationsthat can be performed by the iterative decoder. Consequently, it isdesirable to format FIR samples prior to sending them to the iterativedecoder 414. In this embodiment the iterative decoder 414 does not haveto be aware of the physical sector format, it only needs to deal withthe iterative codeword.

Following FIR filter, the following relationship holds between FIRsamples, and the data that is written onto the media:

$y_{i} = {{\sum\limits_{k = 0}^{M}{h_{k} \cdot \; c_{i - k}}} + {noise}_{i}}$where y_(i) _(—) c_(i) _(—) represents the FIR sample and data bit attime i. Note that the FIR sample at time i is a function of data bits attimes i-M to i. As a result, splicing together FIR samples is not aseasy as splicing hard decisions. In particular, if one simply stitchesFIR samples for data 1 and data 2 portions, i.e. remove the second sync,then the above formula would not necessarily hold around the splicingpoint. This creates a so-called discontinuity in the FIR data stream andcan later result in a degraded iterative decoder error rate. Note thatthe discontinuity does not happen if the last 4 bits of data 1 ismatching the last 4 bits of the 2-nd sync. However this case happenswith a probability of (½)^M, therefore most of the time we do get adiscontinuity.

As discussed previously, there are two main reasons behind logicalsector segmentation on the media—split sector and second sync mark. Inthe following description of the present invention, second sync markwill be used to illustrate various embodiments. However, it should beclear to those skilled in the art that the same techniques describedherein also apply to the case of a split sector without anymodifications.

The process for recovering data will now be described in more detail.

As described above with respect to FIG. 2, ideal FIR data may be givenaccording to the equation

$\begin{matrix}{y_{i}^{\prime} = {\sum\limits_{k = 0}^{M}{h_{k} \cdot c_{i - k}}}} & (1)\end{matrix}$

where y′_(i) is FIR data, c_(k) are codeword bits, h_(k) is a magneticchannel target response of form h=[h₀ h₁ . . . h_(M)], and M is thememory length of a magnetic channel.

As also discussed above, a codeword including a split may be representedby

-   -   c₁c₂c₃ . . . c_(L)a₁a₂a₃ . . . a_(s-2)a_(s-1)a_(s)C_(L+1) . . .        C_(N-1)C_(N)        where (c₁ . . . c_(N)) represent the encoded and interleaved        data bits and where (a₁ . . . a_(s)) represent the split-mark        data bits.

When there is a split in the data introduced following data bit C_(L),the FIR data for M bits after the split-mark end location, i.e. y_(L+1). . . y_(L+M), may be calculated using the following equation, (Eq. 2),which takes the split-mark data into account:

$\begin{matrix}{{y_{L + 1} = {{h_{0} \cdot c_{L + 1}} + {\sum\limits_{k = 1}^{M}{h_{k} \cdot a_{s - k + 1}}} + n_{L + 1}^{y}}}{y_{L + 2} = {{h_{0} \cdot c_{L + 2}} + {h_{1} \cdot c_{L + 1}} + {\sum\limits_{k = 2}^{M}{h_{k} \cdot a_{s - k + 2}}} + n_{L + 2}^{y}}}\vdots{y_{L + M} = {{\sum\limits_{k = 0}^{M - 1}{h_{k} \cdot c_{L + M - k}}} + {h_{M} \cdot a_{s}} + n_{L + M}^{y}}}} & (2)\end{matrix}$where n_(i) ^(y) represents Additive White Gaussian Noise (AWGN) withmean=0 and variance=σ².

Bits c₁c₂ . . . c_(L−1)C_(L) experience the same ISI as if a split-markis not present, due to the causal nature of the ISI. However, bitsc_(L+1)c_(L+2) . . . c_(L+M) experience ISI from codeword bits as wellas from part of the split-mark bits. The split recovery method removesthe ISI caused by the split-mark bits and recovers the sector as if theISI were generated only by the codeword bits (similar to the idealreceived data without a split-mark).

Due to the finite memory of a magnetic channel, only M received samplesafter the split-mark are affected by the split-mark. Thus, split sectorrecovery is performed for these M samples.

FIR data, when the split-mark is present in the channel data, may beexpressed in the form:

-   -   y₁y₂y₃ . . . y_(L)x₁x₂x₃ . . . x_(s-2)x_(s-1)x_(s)y_(L+1) . . .        y_(M)y_(M+1) . . . y_(N-1)y_(N)        where (x₁ . . . x_(s)) represents FIR data corresponding to        split-mark data. At this time, the FIR data includes some data        unaffected by split-mark data because it comes before the split        (y₁, y₂ . . . y_(L)), some data corresponding to split-mark data        (x₁, x₂, . . . x_(s)), some data that is affected by split-mark        data through ISI (y_(L+1), . . . y_(M−1), y_(M)), and some data        unaffected by split-mark data because it is beyond the limit of        the channel memory (y_(M+1), . . . y_(N-1), y_(N)).

From Eq. (1) the ideal FIR data y′_(L+1) can be derived:

$y_{L + 1}^{\prime} = {\underset{\underset{(1)}{︸}}{h_{0} \cdot c_{L + 1}} + \underset{\underset{(2)}{︸}}{\sum\limits_{k = 1}^{M}{h_{k} \cdot c_{L + 1 - k}}}}$

Moreover, the first FIR data corresponding to a split-mark can bewritten as Eq. (3):

$\begin{matrix}{x_{1} = {{{h_{0} \cdot a_{1}} + {\sum\limits_{k = 1}^{M}{h_{k} \cdot c_{L + 1 - k}}} + \left. n_{1}^{x}\Longrightarrow\underset{\underset{(2)}{︸}}{\sum\limits_{k = 1}^{M}{h_{k} \cdot c_{L + 1 - k}}} \right. + n_{1}^{x}} = {x_{1} - {h_{0} \cdot a_{1}}}}} & (3)\end{matrix}$

Using Eq. (2) from above, one can derive Eq. (4):

$\begin{matrix}{{\underset{\underset{(1)}{︸}}{h_{0} \cdot c_{L + 1}} + n_{L + 1}^{y}} = {y_{L + 1} - {\sum\limits_{k = 1}^{M}{h_{k} \cdot a_{s - k + 1}}}}} & (4)\end{matrix}$

Finally, adding Eq. (3) and Eq. (4) yields Eq. (5):

$\begin{matrix}{{\underset{\underset{(1)}{︸}}{h_{0} \cdot c_{L + 1}} + \underset{\underset{(2)}{︸}}{\sum\limits_{k = 1}^{M}{h_{k} \cdot c_{L + 1 - k}}} + \underset{\underset{{two} - {noise} - {terms}}{︸}}{n_{L + 1}^{y} + n_{1}^{x}}} = {y_{L + 1} - {\sum\limits_{k = 1}^{M}{h_{k} \cdot a_{s - k + 1}}} + x_{1} - {h_{0} \cdot a_{1}}}} & (5)\end{matrix}$

The left-side of Eq. (5) is the same as an ideal received signal,y′_(L+1), plus two noise terms. The right side of Eq. (5) has all knownterms or received FIR data terms. Thus, ideal signal recovery for termy_(L+1) is performed using Eq. (5).

In Eq. (5), the ISI contribution due to the split-mark has been removedfrom the FIR data y_(L+1), and the ISI contribution due to the originalcodeword bits has been added to the FIR data y_(L+1), such that itappears as if the split-mark were never present in the sector. In doingso, two noise terms are added to the sample, increasing the noisevariance at the recovered sample positions. However, simulations showthat the benefit of split sector recovery outweighs the increased noisevariance.

In general, the M equations for doing split-sector-recovery may bewritten as:

${y_{L + i}^{\prime} + \underset{\underset{{two} - {noise} - {terms}}{︸}}{n_{L + i}^{y} + n_{i}^{x}}} = {y_{L + i} - {\sum\limits_{k = i}^{M}{h_{k} \cdot a_{s - k + i}}} + x_{i} - {\sum\limits_{k = 0}^{i - 1}{h_{k} \cdot a_{i - k}}}}$

-   -   for 1≦i≦M

Accordingly, using the equation shown above, split sector recovery isperformed for sectors including split-mark data.

FIG. 5 summarizes this process of split recovery.

Item 501 represents the sector FIR data including split-mark data, or

-   -   y₁y₂y₃ . . . y_(L)x₁x₂x₃ . . . x_(s-2)x_(s-1)x_(s)y_(L+1) . . .        y_(M)y_(M+1) . . . y_(N-1)y_(N).

The removal of the split-mark data is represented by item 502. Morespecifically, the split-mark data is removed by first removing FIR datacorresponding to the split-mark, x₁x₂x₃ . . . x₂₋₂x_(s-1)x_(s), shown initem 503.

After removal of the split data, the FIR data with the split-markremoved remains, i.e., y₁y₂y₃ . . . y_(L)y_(L+1) . . . y_(M)y_(M+1) . .. y_(N-1)y_(N) shown in item 504, where y_(L+1) . . . y_(M) is FIR datathat has been affected by the split data because of ISI.

The split sector recovery equation is performed in item 505:

${y_{L + i}^{\prime} + \underset{\underset{{two} - {noise} - {terms}}{︸}}{n_{L + i}^{y} + n_{i}^{x}}} = {y_{L + i} - {\sum\limits_{k = i}^{M}{h_{k} \cdot a_{s - k + i}}} + x_{i} - {\sum\limits_{k = 0}^{i - 1}{h_{k} \cdot a_{i - k}}}}$

-   -   for 1≦i≦M        to subtract out the ISI for M terms following the split-mark.        This operation is specifically utilized by replacing y_(L+),        with the right side of the equation for 1≦i≦M, so that y_(L+1) .        . . y_(M) bits are now equal to the ideal received signal bits        y′_(L+1) . . . y′_(M) from equation (1) plus the two added noise        terms

$\underset{︸}{n_{L + i}^{y} + n_{i}^{x}}.$

Calling the FIR data y″_(L+1) . . . y″_(M), where

${y_{L + i}^{''} = {y_{L + i}^{\prime} + \underset{\underset{{two} - {noise} - {terms}}{︸}}{n_{L + i}^{y} + n_{i}^{x}}}},$the recovered sector data is depicted in item 506 as

-   -   y₁y₂y₃ . . . y_(L)y″_(L+1) . . . y″_(M)y_(M+1) . . .        y_(N-1)y_(N).

FIGS. 6 and 7 show error-location histograms at the SOVA output andouter decoder output, respectively, whereby 600-bit splits were added atthe 1000, 2000, and 3000 bit positions of a codeword of length N>4100bits. As can be seen in these figures, the split sector recovery methodlowers the amount of error at both the SOVA output and outer decoderoutput.

FIGS. 8 and 9 display bit-error-rates at the output of the SOVA andouter decoder output, respectively. These figures show that thebit-error-rate for a codeword containing split data that is processedusing the split sector recovery method will more closely approximate thebit-error-rate of a codeword with no split-marks present than will thebit-error-rate of a codeword containing split data that is simplystitched back together.

Thus, by virtue of the split sector recovery method, SNR loss isordinarily reduced and bit-error rate is ordinarily improved. Moreover,since the system is not required to retain the data corresponding to thesplit-mark data, the amount of required memory can be reduced.

The present disclosure has been described above with respect toparticular illustrative embodiments. It is understood that thedisclosure is not limited to the above-described embodiments and thatvarious changes and modifications may be made by those skilled in therelevant art without departing from the spirit and scope of thisdisclosure.

1. A method comprising: receiving data, wherein the received datacomprises a first segment of user data, a second segment of user data,and non-user data, wherein the non-user data separates the first segmentof user data from the second segment of user data, wherein the non-userdata is affected by a first contribution of inter-symbol interferencedue to the first segment of user data, and wherein the second segment ofuser data is affected by a second contribution of inter-symbolinterference due to the non-user data; removing the non-user data fromthe received data; removing the second contribution of theinter-symbol-interference from the second segment data of user data;adding the first contribution of the inter-symbol-interference to thesecond segment of user data; and subsequent to (i) the removal of thenon-user data, (ii) the removal of the second contribution ofinter-symbol-interference from the second segment data of user data and(iii) the addition of the first contribution ofinter-symbol-interference to the second segment data of user data,stitching together the first segment of user data and the second segmentof user data.
 2. The method of claim 1, wherein the second contributionof inter-symbol-interference is removed from the second segment data ofuser data according to the following equation:${y_{L + i}^{\prime} + \underset{\underset{{two} - {noise} - {terms}}{︸}}{n_{L + i}^{y} + n_{i}^{x}}} = {y_{L + i} - {\sum\limits_{k = i}^{M}{h_{k} \cdot a_{s - k + i}}} + x_{i} - {\sum\limits_{k = 0}^{i - 1}{h_{k} \cdot a_{i - k}}}}$for 1≦i≦M wherein y′ is an ideal signal unaffected by the secondcontribution of inter-symbol-interference, y is the ideal signalaffected by the second contribution of inter-symbol-interference,${\sum\limits_{k = i}^{M}{h_{k} \cdot a_{s - k + i}}} + x_{i} - {\sum\limits_{k = 0}^{i - 1}{h_{k} \cdot a_{i - k}}}$represents the second contribution of inter-symbol-interference over ‘M’bits of the second segment of user data, ‘h’ represents a magneticchannel target response, ‘a’ represents data bits of the non-user data,and ‘x’ represents data bits of the non-user data after convolutionthrough the magnetic channel target response.
 3. The method of claim 1,wherein the non-user data is split mark data.
 4. The method of claim 1,wherein the non-user data is servo data.
 5. The method of claim 1,wherein the received data is produced by a convolution of (i) channelbits and (ii) a magnetic channel target response.
 6. The method of claim1, further comprising: subsequent to the first segment of user data andthe second segment of user data being stitched together, transmittingthe first segment of user data and the second segment of user data to aViterbi detector.
 7. A tangible computer-readable storage mediumcomprising instructions tangibly stored thereon, the instructions beingexecutable by a programmable processor to: receive data, wherein thereceived data comprises a first segment of user data, a second segmentof user data, and non-user data, wherein the non-user data separates thefirst segment of user data from the second segment of user data, whereinthe non-user data is affected by a first contribution of inter-symbolinterference due to the first segment of user data, and wherein thesecond segment of user data is affected by a second contribution ofinter-symbol interference due to the non-user data; remove the non-userdata from the received data; remove the second contribution of theinter-symbol-interference from the second segment data of user data; addthe first contribution of the inter-symbol-interference to the secondsegment of user data; and subsequent to (i) the removal of the non-userdata from the received data (ii) the removal of the second contributionof inter-symbol-interference from the second segment data of user dataand (ii) the addition of the first contribution ofinter-symbol-interference to the second segment data of user data,stitch together the first segment of user data and the second segment ofuser data.
 8. The tangible computer-readable storage medium of claim 7,wherein the second contribution of inter-symbol-interference is removedfrom the second segment of user data according to the followingequation:${y_{L + i}^{\prime} + \underset{\underset{{two} - {noise} - {terms}}{︸}}{n_{L + i}^{y} + n_{i}^{x}}} = {y_{L + i} - {\sum\limits_{k = i}^{M}{h_{k} \cdot a_{s - k + i}}} + x_{i} - {\sum\limits_{k = 0}^{i - 1}{h_{k} \cdot a_{i - k}}}}$for 1≦i≦M wherein y′ is an ideal signal unaffected by the secondcontribution of inter-symbol-interference, y is the ideal signalaffected by the second contribution of inter-symbol-interference,${\sum\limits_{k = i}^{M}{h_{k} \cdot a_{s - k + i}}} + x_{i} - {\sum\limits_{k = 0}^{i - 1}{h_{k} \cdot a_{i - k}}}$represents the second contribution of inter-symbol-interference over ‘M’bits of the second segment of user data, ‘h’ represents a magneticchannel target response, ‘a’ represents split-mark data bits, and ‘x’represents the split-mark data bits after convolution through themagnetic channel target response.
 9. The tangible computer-readablestorage medium of claim 7, wherein the received data is produced by aconvolution of (i) channel bits and (ii) a magnetic channel targetresponse.
 10. The tangible computer-readable storage medium of claim 7,wherein the instructions are further being executable by theprogrammable processor to: subsequent to the first segment of user dataand the second segment of user data being stitched together, transmitthe first segment of user data and the second segment of user data to aViterbi detector.
 11. A disk drive controller comprising: a reading unitconfigured to read encoded data from a hard disk, wherein the encodeddata comprises a first segment of data, a second segment of data, andsplit-mark data, wherein the split-mark data separates the first segmentof data from the second segment of data, wherein the split-mark data isaffected by a first contribution of inter-symbol interference due to thefirst segment of data, and wherein the second segment of data isaffected by a second contribution of inter-symbol interference due tothe split-mark data; an identification unit configured to identify, inthe encoded data, the first segment of data, the second segment of data,and the split-mark data; a recovery unit configured to (i) remove thesplit-mark data from the encoded data, (ii) remove the secondcontribution of the inter-symbol-interference from the second segmentdata of data, and (iii) add the first contribution of theinter-symbol-interference to the second segment of data; and a stitchingunit configured to, subsequent to the recovery unit (i) removing thesplit-mark data (ii) removing the second contribution of theinter-symbol-interference and (iii) adding the first contribution of theinter-symbol-interference, stitch together the first segment of data andthe second segment of data.
 12. The disk drive controller of claim 11,further comprising: an output unit configured to, subsequent to thestitching unit stitching together the first segment of data and thesecond segment of data, transmit the first segment of data and thesecond segment of data to a decoder.
 13. The disk drive controller ofclaim 11, wherein the second contribution of inter-symbol-interferenceis removed from the second segment of data according to the followingequation:${y_{L + i}^{\prime} + \underset{\underset{{two} - {noise} - {terms}}{︸}}{n_{L + i}^{y} + n_{i}^{x}}} = {y_{L + i} - {\sum\limits_{k = i}^{M}{h_{k} \cdot a_{s - k + i}}} + x_{i} - {\sum\limits_{k = 0}^{i - 1}{h_{k} \cdot a_{i - k}}}}$for 1≦i≦M wherein y′ is an ideal signal unaffected by the secondcontribution of inter-symbol-interference, y is the ideal signalaffected by the second contribution of inter-symbol-interference,${\sum\limits_{k = i}^{M}{h_{k} \cdot a_{s - k + i}}} + x_{i} - {\sum\limits_{k = 0}^{i - 1}{h_{k} \cdot a_{i - k}}}$represents the second contribution of inter-symbol-interference over ‘M’bits of the second segment of data, ‘h’ represents a magnetic channeltarget response, ‘a’ represents split-mark data bits, and ‘x’ representsthe split-mark data bits after convolution through the magnetic channeltarget response.
 14. The disk drive controller of claim 11, wherein theencoded data is produced by a convolution of (i) channel bits and (ii)magnetic channel target response.